An interesting side note is that the raphe nuclei are a core site of activity for LSD....
Low levels of serotonin in the brain are known to play a role in depression and anxiety, and it is customary to treat these disorders with medications that increase the amount of this neurotransmitter. However, a new study carried out by researchers at Columbia University Medical Center (CUMC) suggests that this approach may be too simple. It appears that neighboring serotonin-producing brainstem regions exert different and sometimes opposing effects on behavior.
From anatomical studies, researchers knew that the brainstem contains two distinct clusters of serotonergic neurons: one in dorsal raphe nucleus (DRN) and another in the median raphe nucleus (MRN). Together both regions harbor the vast majority of neurons that supply serotonin to the rest of the brain, but it was unclear how neuronal activity within these clusters controls behavior.
"Going into the study, our hypothesis was that reduced activity of serotonergic neurons is what drives these mood behaviors," said Dr. Ansorge. "But what we found was more complicated. First, it appears that hyperactivity of the MRN drives anxiety-like behavior. We also observed that decreased DRN activity increases depression-like behavior, while decreased MRN activity reduces it. This led us to conclude that an imbalance between DRN and MRN activity is what leads to depression-like behavior."